The present invention relates generally to plasma process chambers, and more particularly, to methods and systems for applying a desired power input to the plasma.
Variability in RF power delivery to a plasma reactor chamber can be caused by RF current flowing through components which absorb RF power at locations downstream of the calibration location. In plasma processing of semiconductor wafers, the variability in RF power delivered to the plasma and associated process shifts causes process variability from a first plasma chamber to a second plasma chamber in the absorption properties of RF current carrying components. Semiconductor wafer to wafer variability may also result due to absorption properties changing over time, or due to temperature fluctuations.
Typically, line loss measurements are used to calibrate RF generator power and account for RF losses in cables, etc. between the RF power generator and the “antenna” (e.g., RF coil). However, typical calibration methods can only measure power delivery at the input to the impedance matching network, and are unable to compensate for variable power dissipation at locations within the match network, within the electrode or coil assemblies, or in shielding or other components which carry mirror currents. As a result, typical calibration methods can not ensure accurate power delivery into the plasma.
The problem is can be particularly acute when RF current flows on or near components having a magnetic permeability higher than vacuum permeability, or a relatively high resistivity (i.e. higher than copper). Materials known to be associated with variable absorption include, but are not limited to nickel plating, high magnetic permeability alloys such as mu-metal and materials with temperature or time dependent electrical resistivity. Nickel plating is commonly used on RF components in conjunction with silver plating for mil-spec silver plating processes.
As the power actually applied to the plasma in each of a first plasma processing chamber and a second plasma processing chamber is unknown, the first plasma processing chamber and the second plasma processing chamber will not be identical. What is needed is a system, method and apparatus for determining the power actually applied to the plasma so that the plasma in a first plasma processing chamber can be much more closer to identical to the plasma in a second plasma processing chamber.